This invention relates in general to algorithms for anti-lock brake systems and in particular to a control algorithm for detecting wheel speed sneakdown on a low mu surface.
Braking a vehicle in a controlled manner under adverse weather conditions, such as rain, snow or ice, generally requires precise application of the vehicle wheel brakes by the vehicle operator. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure which causes the vehicle wheels to lock-up such that excessive slippage between the wheels and the road surface takes place. Wheel lock-up conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout. Accordingly, an Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of the vehicle wheel brakes to prevent lock-up of the associated vehicle wheels. One type of ABS controls only the rear vehicle wheel brakes. Such a system is referred to as a RWAL in the following description.
A typical prior art RWAL system 10 is illustrated in FIG. 1. The RWAL system 10 includes a normally open solenoid valve 22 connected between the vehicle master cylinder 14 and the controlled rear wheel brakes 20a and 20b. When actuated, the normally open solenoid valve 22 closes to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. Accordingly, the normally open solenoid valve 22 is referred to below as an isolation valve. The isolation valve 22 also can be selectively opened to increase the pressure at the rear wheel brakes 20a and 20b. The RWAL system 10 also includes a normally closed solenoid valve 26, which is referred to below as a dump valve. The dump valve 26 is selectively opened to reduce the pressure at the rear wheel brakes by bleeding brake fluid from the rear wheel brakes 20a and 20b to an accumulator 28. The isolation and dump valves 22 and 26 are mounted within a control valve 21.
The vehicle brake system master cylinder 14 provides a source of pressurized hydraulic brake fluid to the RWAL system 10. Thus, a separate hydraulic source, such as a motor driven pump, which is usually included in a four wheel ABS, is not needed. This reduces the complexity and cost of manufacturing the RWAL system 10, which is typically referred to as a passive system. The RWAL system 10 further includes an electronic control module 30 which is electrically connected to a wheel speed sensor 40 and to the isolation and dump valves 22 and 26. The control module 30 can be mounted directly upon the control valve 21 or located remotely therefrom.
The control module 30 includes a microprocessor (not shown) which is programmed to control the RWAL system in accordance with a control algorithm and parameters permanently stored in a Read Only Memory (ROM). Typically, the control algorithm is trimmed for the particular vehicle in which the ABS is installed. The microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL system 10 illustrated in FIG. 1 is included in U.S. Pat. Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the ABS electronic control module 30 continuously receives speed signals from the wheel speed sensor 40. During a vehicle braking cycle, the ABS microprocessor monitors the rear wheel speed and deceleration. The microprocessor calculates a theoretical speed ramp, which represents the speed the vehicle would travel if decelerated at a predetermined maximum rate, such as, for example, 1.0 g. The microprocessor compares the actual rear wheel speed to the theoretical ramp. If the rear wheel deceleration reaches a predetermined value, such as, for example, 1.3 g, the microprocessor determines that the rear wheel brakes 20a and 20b may be approaching a rear wheel lock-up condition. Accordingly, the ABS microprocessor closes the isolation valve 22 to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. If the rear wheel speed departs form the theoretical ramp in addition to, or in place of, the deceleration condition, the ABS microprocessor determines that the rear wheel brakes 20a and 20b are certainly approaching a lock-up condition and the microprocessor maintains the isolation valve 22 in the closed position. The ABS microprocessor then selectively opens the dump valve 26 to reduce the pressure applied to the rear wheel brakes 20a and 20b to correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve to initiate a second wheel speed departure to adjust the rear wheel brake pressure upward.
The operation of the RWAL system is illustrated by the graphs shown in FIG. 2. The upper curve shows the rear wheel speed as a function of time while the lower curve shows the rear wheel brake pressure as a function of time. The middle curves illustrate the operation of the isolation and dump valves 22 and 26 as a function of time. The solid curve labeled 60 represents the velocity of the rear wheels while the dashed curve labeled 64 represents the vehicle velocity. The first and second wheel speed departures are labeled 60a and 60b, respectively. Following correction of the second wheel speed departure, which occurs at time t7, the rear wheel brake pressure is maintained a constant level Pe, as shown in the lower curve.
If the vehicle transitions from a low mu to a high mu road surface, a key feature included in the algorithm utilized by the RWAL system 10 is that the braking effort exerted by the rear wheel brakes 20a and 20b can be increased to utilize the increased mu. An example of such a transition is shown at t8 in FIG. 2. The transition can be detected by monitoring the deceleration of rear wheels which can increase due to the greater braking effect of the uncontrolled front wheel brakes 19a and 19b upon the higher mu road surface. If the rear wheel deceleration increases by a predetermined amount, such as 1.0 g, the microprocessor assumes that the change is due to the road surface transition and reopens the isolation valve 22 to generate an unlimited series of reapply pulses 62b. The resulting increased pressure to the rear wheel brakes 20a and 20b initiates a third wheel speed departure, which is labeled 60c in FIG. 2. At time t10, a dump pulse is generated to open the dump valve 26 to reduce the rear wheel brake pressure to a level Pg to correct the third rear wheel departure. Thereafter, the rear wheel brake pressure is held at the level Pg, which is greater than the previously held level Pe.
This invention relates to an improved control algorithm for an anti-lock brake system which detects wheel speed sneakdown on a low mu surface.
During an anti-lock brake cycle, it is possible for the rear wheel speed to follow an overall trajectory approaching 1.0 g even though the friction coefficient of the road surface may be in the neighborhood of only 0.1. This condition is often referred to as wheel speed sneakdown. Wheel speed sneakdown can occur gradually or following a wheel speed excursion. An example of wheel speed sneakdown occurring following a wheel speed excursion is shown in FIG. 3 where the solid line represents the rear wheel speed and the dashed line represents the vehicle speed. Similar to FIG. 2, at t6 a second wheel speed excursion is initiated. At t14, the wheel speed departure and recovery cycle appears to the ABS microprocessor to have been completed, causing the microprocessor to decide that the rear wheel speed has returned to the vehicle speed and that the wheel speed excursion has ended. Actually, the rear wheel is following a wheel speed curve approximating 1.0 g. Accordingly, when microprocessor samples the rear wheel speed, the microprocessor will determine that a low-to-high road surface transition has occurred. The microprocessor algorithm will then initiate a third wheel speed departure with an unlimited series of reapply pulses even though the vehicle is still on a low mu road surface. The resulting wheel speed departure cycle is wasteful of the limited amount of pressurized brake fluid available in the master cylinder 14.
While the above example is for a second wheel speed excursion, it will be appreciated that wheel speed sneakdown also may occur during an anti-lock braking cycle following the first or any subsequent wheel speed excursion. Wheel speed sneakdown can occur in any ABS not having a G-sensor, but is most severe for RWAL systems, which have only one speed sensor 40. Thus, it would be desirable to detect the presence of wheel speed sneakdown to avoid initiating an unneeded wheel speed departure.
The present invention contemplates a system for controlling at least one vehicle wheel brake which includes a valving device connected to the controlled vehicle wheel brake. The valving device being operable to control application of pressurized fluid to the controlled wheel brake. The system also includes a wheel speed sensor for monitoring the speed of a vehicle wheel associated with the controlled wheel brake. The system further includes a microprocessor coupled to the valving device and the so wheel speed sensor.
The microprocessor is operative to selectively actuate the valving device to control the wheel brake. The microprocessor also is operative to measure a duration of a wheel speed departure of the wheel associated with the controlled wheel brake and to calculate an average deceleration of the wheel during the wheel speed departure. The microprocessor is further operative to set a wheel speed sneakdown flag if the wheel speed departure duration is greater than or equal to a predetermined wheel speed departure duration threshold and the average deceleration greater than or equal to a deceleration threshold.
After setting said sneakdown flag, the microprocessor is further operative to cause the valving device to decrease the pressure applied to the controlled wheel brake. Following the decrease of pressure applied to the controlled wheel brake, the microprocessor can cause the valving device to increase the pressure applied to the controlled wheel brake. Alternately, if the wheel speed departure duration is less than said predetermined wheel speed departure duration threshold, the microprocessor can cause the valving device to increase the pressure applied to the controlled wheel brake. Furthermore, if the average deceleration is less than the deceleration threshold, the microprocessor can cause the valving device to maintain the pressure applied to the controlled wheel brake.
In the preferred embodiment, the deceleration threshold is non-linear function of the wheel speed departure. However, the deceleration threshold also can be predetermined constant or a linear function of the wheel speed departure duration. Also, in the preferred embodiment, the controlled wheel brake is a rear wheel brake and the system is included in a rear wheel anti-lock brake system.
The present invention also contemplates a method for controlling at least one vehicle brake which includes providing a valving device for controlling the application of a pressurized fluid to the controlled wheel brake and a wheel speed sensor for monitoring the speed of a vehicle wheel associated with the controlled wheel brake. Both the valving device and wheel speed sensor are coupled to a microprocessor. The microprocessor measures the duration of a wheel speed departure of the wheel associated with the controlled wheel brake and computes an average deceleration for the wheel during the wheel speed departure duration. The microprocessor then compares the wheel speed departure duration to a predetermined wheel speed departure duration threshold and the average deceleration to a deceleration threshold. The microprocessor sets a wheel speed sneakdown flag if the wheel speed departure duration is greater than or equal to the wheel speed departure duration threshold and the average deceleration is greater than or equal to the deceleration threshold.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.